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I would like to use some C++ std compliant memory management in form of a class derived from std::allocator, but able to allocate chunks of memory and releasing & freeing them in smaller parts. I only found boost::pool, but this is not std compliant in the above sense. Is there anything more useful around or do I have to code this myself?

(Note that the std::allocator is often useless for allocating many small objects, i.e. when using a std::list.)

EDIT to clarify.

Say, I want to use a std::list of many small objects, then implementations of std::allocator which allocate each object using ::new cause significant overhead in run time (but also memory I think). It is much more efficient to allocate big chunks of objects and hand them out one by one. For this, I need a std-compliant allocator (doesn't need to be derived from std::allocator, but must implement the same concept) that can be used with any std library container and provides the required memory management, ideally allowing me to tell it how many objects I am likely to individually allocate.

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    What is the issue with Boost.Pool allocators? Why do you say they aren't standard compliant? Commented Jun 20, 2012 at 10:48
  • Please specify if you have any particular allocation/free requirements. Commented Jun 20, 2012 at 11:13
  • @Karthik and @interjay Let's assume I want to use std::list (as mentioned in the original question) but avoid the allocation of many single objects. Commented Jun 20, 2012 at 11:44
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    "in form of a class derived from std::allocator" N.B. there is no reason allocators must (or should) derive from std::allocator, it's not an abstract base or an interface class. Allocators only have to meet the allocator requirements (17.6.3.5 [allocator.requirements]) not inherit from a particular base class. Commented Jun 20, 2012 at 12:19
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    Also, what do you mean by "the std::allocator is often useless" ... do you mean your compiler's implementation? Or all implementations? They're not all the same. Commented Jun 20, 2012 at 12:31

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GCC provides a few extension allocators as alternatives to std::allocator.

You haven't really said what your requirements are, so it's not possible to say if any of them would be suitable for you.

Edit following OP's edit:

Say, I want to use a std::list of many small objects, then implementations of std::allocator which allocate each object using ::new cause significant overhead in run time (but also memory I think).

Why also memory? The overhead of additional pointers in each std::list node will be present whether the memory comes from new or a custom allocator. Do you just mean the bookkeeping done by the heap to track all the small allcoations?

It is much more efficient to allocate big chunks of objects and hand them out one by one.

Have you measured it?

If you don't want the overhead of allocating lots of separate nodes are you sure std::list is the right container? What about vector or deque?

boost::stable_vector is still node-based but has less per-node memory overhead than std::list.

A boost::flat_map<int, T> isn't node-based and could be used instead of std::list<T>

Allocators are tricky and not always the best answer to (real or perceived) problems.

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2 Comments

+1 for the hint of the gcc extension. What do you use? std::allocator for everything (are you after performance?)
Yes, usually just std::allocator for containers, custom new/delete for performance-sensitive allocation.

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